Solvolysis of 2-aryl-2-chloro-4,4-dimethylpentanes. Confirmation of validity of brown-okamoto σ+ constants

Article abstract of DOI:10.1021/jo9821067

Specific rate constants of solvolysis in 90% aqueous acetone have been determined at 25 °C for 10 2-aryl-2-chloro-4,4-dimethylpentanes (3) having a substituent p-cycPr, p-Me, m-Me, p-F, (H), p-Cl, m-Cl, m-CF₃, p-CF₃, or p- NO₂

Full text of DOI:10.1021/jo9821067

J . Org. Chem. 1999, 64, 2375-2380
2375
Solvolysis of 2-Ar yl-2-ch lor o-4,4-d im eth ylp en ta n es. Con fir m a tion of
Va lid ity of Br ow n -Ok a m oto σ+ Con sta n ts
Ken’ichi Takeuchi,* Masaaki Takasuka, Yasushi Ohga, and Takao Okazaki
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University,
Sakyo-ku, Kyoto 606-8501, J apan
Received October 19, 1998
Specific rate constants of solvolysis in 90% aqueous acetone have been determined at 25 °C for 10
2-aryl-2-chloro-4,4-dimethylpentanes (3) having a substituent p-cycPr, p-Me, m-Me, p-F, (H), p-Cl,
m-Cl, m-CF3, p-CF3, or p-NO2. The Hammett-Brown relation is satisfactorily linear (r ) 0.999)
with F+ of -4.51, showing that Brown-Okamoto σ+ values based on the solvolysis of the cumyl
chloride system 1 are widely usable for the substrates solvolyzing without appreciable nucleophilic
solvent participation. Solvolysis of 3 having 3,5-Cl2 or 3,5-(CF3)2 substituents suggests the use of
σ+ values of +0.740 and +0.982 instead of respective 2σm+ values of +0.798 and +1.040. The large
negative F+ values in TFE of -5.91 and -5.63 for 1 and 3, respectively, reflect decreased nucleophilic
(or Brønsted base type) solvation. Comparison of solvolysis rates of 1 with 3, both having m-Cl,
p-CF3, or 3,5-(CF3)2 substituents, in aqueous ethanol, aqueous acetone, TFE, and TFE-EtOH
suggests that nucleophilic intervention by solvent in the cumyl system increases as the intermediate
carbocation becomes more electron-demanding.
In tr od u ction
activated complex for ionization.2 Meantime, Richard et
al. suggested that the Brønsted base type solvation to
the methyl hydrogen atoms of the cumyl cation would
be more important than the Lewis base type solvent
intervention toward the cationic carbon.3 It was cautioned
that, whatever the mechanism of the nucleophilic solvent
intervention may be, it would introduce an undesirable
perturbation of the σ+ values, especially if it varied with
the substituent.4
The extent of nucleophilic solvent intervention in the
cumyl solvolysis has been semiquantitatively examined
on the basis of extended Grunwald-Winstein relations2,4
and comparison of cation stability between the solution
and gas phases.3b However, the effect of nucleophilic
solvent intervention on the validity of the Brown-
Okamoto σ+ values has never been clarified.
One of the most important structure-reactivity rela-
tionships in organic chemistry is the Hammett equation
(eq 1), where k (or K) and k0 (or K0) are the specific rate
(or equilibrium) constants of a given compound having a
substituent and of an unsubstituted one, respectively, σ
is a substituent constant that is defined by eq 2, (where
K and K0 are dissociation constants of substituted and
unsubstituted benzoic acids, respectively, in water at 25
°C) and F is a reaction constant.
log(k/k0) ) Fσ
log(K/K0) ) σ
(1)
(2)
In the 1950s, Brown noticed that the Hammett relation
holds for the rates of electrophilic aromatic substitutions
and solvolysis of substituted cumyl chlorides (1) only
when the ring substituent is located in the meta position
and defined new substituent constants (σ+) based on the
specific rates of solvolysis of cumyl chloride and its ring
substituted homologues (k0 and k, respectively) in 90%
acetone-10% water (v/v) at 25 °C (eq 3).1 The F+ value
of -4.54 was defined by using unsubstituted and meta
substituted cumyl chlorides.1
Recently, we have studied the Grunwald-Winstein
relationship for 2-chloro-2,4,4-trimethylpentane (2)5a,b
and related systems,5c and reported that 2 is a tertiary
alkyl kc substrate that solvolyzes practically without
either nucleophilic solvent intervention or neighboring
group participation. The essential vanishment of nucleo-
philic solvent intervention has been ascribed to an
efficient rear-side shielding effect of the neopentyl sub-
(2) (a) Liu, K.-T.; Sheu, H.-C.; Chen, H.-I.; Chiu, P.-F.; Hu, C.-R.
Tetrahedron Lett. 1990, 31, 3611. (b) Liu, K.-T.; Sheu, H.-C. J . Org.
Chem. 1991, 56, 3021. (c) Liu, K.-T.; Chen, H.-I.; Chin, C.-P. J . Phys.
Org. Chem. 1991, 4, 463. (d) Liu, K.-T.; Chang, L.-W.; Chen, P.-S. J .
Org. Chem. 1992, 57, 4791. (e) Liu, K.-T.; Chen, P.-S.; Chiu, P.-F.; Tsao,
M.-L. Tetrahedron Lett. 1992, 33, 6499. (f) Liu, K.-T.; Chen, P.-S.; Hu,
C.-R.; Sheu, H.-C. J . Phys. Org. Chem. 1993, 6, 122.
(3) (a) Richard, J . P.; Amyes, T. L.; Vontor, T. J . Am. Chem. Soc.
1991, 113, 5871. (b) Richard, J . P.; J agannadham, V.; Amyes, T. L.;
Mishima, M.; Tsuno, Y. J . Am. Chem. Soc. 1994, 116, 6706.
(4) (a) Kevill, D. N. In Advances in Quantitative Structure-Property
Relationships; Charton, M., Ed.; J AI Press: Greenwich, CT, 1996; Vol.
1, pp 81-115. (b) Kevill, D. N.; D’Souza, M. J . J . Chem. Res., Synop.
1993, 332.
(-1/4.54)log(k/k0) ) σ+
(3)
The so-called Brown-Okamoto σ+ constants have
made enormous contribution to organic chemistry. How-
ever, recent studies by Liu and co-workers indicated the
possibility that the solvolysis of cumyl chloride and its
substituted homologues may not be a kc process, but one
assisted by nucleophilic solvent participation in the
(5) (a) Takeuchi, K.; Ohga, Y.; Ushino, T.; Takasuka, M. J . Phys.
Org. Chem. 1996, 9, 777. (b) Takeuchi, K.; Ohga, Y.; Ushino, T.;
Takasuka, M. J . Phys. Org. Chem. 1997, 10, 717. (c) Takeuchi, K.;
Ohga, Y.; Ushino, T.; Takasuka, M. J . Org. Chem. 1997, 62, 4904.
* Tel: +81 75 753 5689. FAX: +81 75 753 3350. E-mail: ktake@
scl.kyoto-u.ac.jp.
(1) Brown, H. C.; Okamoto, Y. J . Am. Chem. Soc. 1958, 80, 4979.
10.1021/jo9821067 CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/27/1999

2376 J . Org. Chem., Vol. 64, No. 7, 1999
Takeuchi et al.
Ta ble 1. Ca lcu la ted An gles of Tw istin g (d eg) fr om
Cop la n a r ity betw een th e Ar yl Rin g a n d th e sp 2 P la n e of
Ca r boca tion s 1e+, 3e+, a n d 4e+ on th e Ba sis of
RHF /6-31G*
Sch em e 1
angle of twisting (deg)
source
1e+
3e+
4e+
this work
ref 8b
5.0
5
3.2
24.5
24
ref 7
23.8
1
by H NMR showed the formation of 2-[(p-trifluorometh-
yl)phenyl]-4,4-dimethyl-2-pentanol (29%) and 2-[(p-trif-
luoromethyl)phenyl]-4,4-dimethyl-1-pentene (66%) as ma-
jor products. The small signals at δ 2.16, which may be
assigned to the methyl groups of (Z)- and (E)-2-[(p-
trifluoromethyl)phenyl]-4,4-dimethyl-2-pentene, suggested
their formation in 5%.
Ca lcu la ted Str u ctu r es of Ca r boca tion s. For the
examination of Brown-Okamoto σ+ constants, it is a
prerequisite that the delocalization of positive charge to
the para position is similar between the two systems 1
and 3 in the activated complex for ionization in solvolysis.
To satisfy the prerequisite, the two systems having an
identical substituent must attain similar coplanarity
between the aryl ring and the sp2 plane of the cationic
center. Recently, experimental and theoretical studies on
resonance demand in benzylic carbocations have been
actively carried out by the groups of Liu and Tsuno.7,8
We calculated the structure of unsubstituted carbocation
3e+ by using the RHF/6-31G* basis set.9 For a compara-
tive purpose, we repeated the previously reported calcu-
lations on 1e+ and 4e+; the twisted angles between the
stituent.5 Consequently, we decided to examine whether
the Hammett-Brown relation holds for the solvolysis of
2-aryl-2-chloro-4,4-dimethylpentanes (3) that were ex-
pected to behave as kc substrates. We further wished to
examine the additivity of the σ+ values for the m-Cl and
m-CF3 substituents by using the solvolysis of 3k and 3l.
phenyl ring and the sp2 cationic plane for the three
carbocations are summarized in Table 1. The angles for
1e+ and 4e+ calculated in this work are 5.0° and 24.5°,
respectively, and are in good agreement with reported
values (5° 8b and 24° 8b or 23.8° 7).10 Notably, the twisted
angle for 3e+ is 3.2°, which is even smaller than that for
1e+. This suggests that the system 3 is an appropriate
model to examine the validity of σ+ constants in the
essential absence of nucleophilic solvent intervention.
This notion was also supported by the para/ meta sol-
volysis rate ratios for CH3 and CF3 substituents as an
index for resonance contribution (see below).
Resu lts a n d Discu ssion
Syn th esis. The chlorides 3 were prepared by hydro-
chlorination6 of the corresponding alcohols that were
derived by the Grignard reaction of 4,4-dimethyl-2-
pentanone with arylmagnesium bromides. In some cases
(m-Cl, m-CF3, and 3,5-(CF3)2), the alcohols were dehy-
drated to olefins, which were then hydrochlorinated. The
p-nitro derivative 3j was prepared by the route shown
in Scheme 1. The purities of freshly prepared 3e-l were
generally 70-97% by titration, the rest being the starting
alcohol or olefins as estimated by 13C NMR measure-
ments. However, the chlorides 3a -d were very unstable
and formed large amounts of olefins during hydrochlo-
rination; therefore, they were used directly for rate
measurements immediately after evaporation of solvent.
Solvolysis P r od u cts. As a substrate for product
study, p-trifluoromethyl derivative 3i was selected be-
cause it could be prepared in a relatively pure state. The
solvolysis was conducted on 3i (0.02 mol L-1) in 90%
(7) Liu, K.-T.; Tsao, M.-L.; Chao, I. Tetrahedron Lett. 1996, 37, 4173.
(8) (a) Nakata, K.; Fujio, M.; Saeki, Y.; Mishima, M.; Tsuno, Y.;
Nishimoto, K. J . Phys. Org. Chem. 1996, 9, 561. (b) Nakata, K.; Fujio,
M.; Saeki, Y.; Mishima, M.; Tsuno, Y.; Nishimoto, K. J . Phys. Org.
Chem. 1996, 9, 573.
(9) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
J ohnson, B. G.; Robb, M. A.; Cheeseman, J . R.; Keith, T.; Petersson,
G. A.; Montgomery, J . A.; Raghavachari, K.; Al-Laham, M. A.;
Zakrzewski, V. G.; Ortiz, J . V.; Foresman, J . B.; Cioslowski, J .;
Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala,
P. Y.; Chen, W.; Wong, M. W.; Andres, J . L.; Replogle, E. S.; Gomperts,
R.; Martin, R. L.; Fox, D. J .; Binkley, J . S.; Defrees, D. J .; Baker, J .;
Stewart, J . P.; Head-Gordon, M.; Gonzalez, C.; Pople, J . A. Gaussian
94, Revision D.3; Gaussian, Inc.: Pittsburgh, PA, 1995.
acetone in the presence of 2,6-lutidine (0.025 mol L-1
for 10 half-lives at 25 °C. Analysis of the crude product
)
(10) A recent X-ray crystallographic study showed the twisted angle
of 8(2)° for the cumyl cation of cumyl hexafluoroantimonate; see:
Laube, T.; Olah, G. A.; Bau, R. J . Am. Chem. Soc. 1997, 119, 3087.
(6) Brown, H. C.; Rei, M.-H. J . Org. Chem. 1966, 31, 1090.

Solvolysis of 2-Aryl-2-chloro-4,4-dimethylpentanes
J . Org. Chem., Vol. 64, No. 7, 1999 2377
Ta ble 2. Br ow n -Ok a m oto σ+ Con sta n ts a n d Sp ecific
Ra te Con sta n ts for th e Solvolysis of
Ta ble 3. Sp ecific Ra te Con sta n ts for th e Solvolysis of
3g, 3i, 3l, a n d 1l in Va r iou s Solven ts a t 25 °Ca
2-Ar yl-2-ch lor o-4,4-d im eth ylp en ta n es (3) in 90% Aqu eou s
k
c (s-1), substituent and substrate
Aceton e a t 25 °Ca
solventb
m-Cl, 3g
p-CF3, 3i
3,5-(CF3)2, 3l 3,5-(CF3)2, 1l
substituent
substrate
σ+ b
k (s-1
)
d
h
h
h
h
d
d
h
h
h
d
d
d
d
h
e,f
e,g
e,i
e,j
d
100E
90E
80E
70E
60E
100M
90A
80A
2.39 × 10-4
1.79 × 10-3
6.62 × 10-3
1.92 × 10-2
5.86 × 10-2
2.12 × 10-5
1.98 × 10-4
7.64 × 10-4
2.01 × 10-3
5.48 × 10-3
2.18 × 10-4
1.16 × 10-5
1.00 × 10-4
4.70 × 10-4
2.16 × 10-3
3.1 × 10-7
1.3 × 10-8
9.1 × 10-8
3.5 × 10-7
p-cycPr
p-Me
m-Me
p-F
H
p-Cl
3a
3b
3c
3d
3e
3f
3g
3h
3i
-0.462c
-0.311
-0.10
-0.073
0.000
0.11
0.399
0.520
0.612
0.790
8.5 × 10-1 d,e
d
d
d
1.66 × 10-6
5.03 × 10-6
1.24 × 10-5
1.9 × 10-1 f,g
(1.54 ( 0.02) × 10-2 f
(1.35 ( 0.02) × 10-2 f
(7.27 ( 0.02) × 10-3 f
(2.06 ( 0.01) × 10-3 h
1.18 × 10-4 h
9.74 × 10-7
d
d
k
1.18 × 10-4
2.53 × 10-7
6.3 × 10-9
m-Cl
m-CF3
p-CF3
p-NO2
3,5-Cl2
3,5-(CF3)2
2.75 × 10-5 h
70A
60A
100T
1.16 × 10-5 h
e,l
e,m
h
h
d
d
d
d
2.00 × 10-6 h
2.7
2.5 × 10-1
1.43 × 10-3
1.74 × 10-4
2.61 × 10-5
6.67 × 10-6
1.26 × 10-6
3j
3k
3l
e,n
h
80T-20E 3.1 × 10-1
3.07 × 10-2
3.11 × 10-6 h
h
e,o
60T-40E 4.67 × 10-2
4.24 × 10-3
2.9 × 10-7
2.5 × 10-7 d,i
a
a
b
The specific rate constants for 3g, 3i, and 3l in 90A were taken
from Table 2. E, M, A, and T denote ethanol, methanol, acetone,
The solvolysis was conducted in the absence of buffer. Taken
b
from ref 1, except the value for p-cycPr. c Ref 11. Extrapolated
d
from data at other temperatures. eConductometric specific rate
and 2,2,2-trifluoroethanol, respectively, and the preceding figures
denote vol % of the organic components in aqueous mixtures at
25 °C except for T-E solvent systems. The figures given for the
T-E systems indicate respective vol % at 25 °C. c Except for 90A,
the rates were determined in the presence of 2,6-lutidine; 2 × 10-4
mol L-1 in T and T-E systems and 0.025 mol L-1 in the other
solvents. Most of the data were obtained from duplicate runs
within experimental errors of (2% and (0.5% in titrimetric and
constants were 0.0172 s-1 (-20.0 °C), 0.0420 s-1 (-10.5 °C), and
0.111 s-1 (-0.4 °C). f Determined conductometrically. Conduc-
g
tometric specific rate constants were 0.0253 s-1 (2.5 °C), 0.0532
s-1 (11.5 °C), 0.0806 s-1 (16.0 °C), and 0.0823 s-1 (16.2 °C).
h
i
Determined titrimetrically. Titrimetrically determined specific
rate constants were 4.43 × 10-6 s-1 (50.0 °C), 5.13 × 10-5 s-1 (75.0
°C), and 3.95 × 10-4 s-1 (100.0 °C).
d
conductometric runs, respectively. Determined titrimetrically.
e Extrapolated from data at other temperatures. f Other specific
rate constants were 7.26 × 10-6 s-1 (50.0 °C) and 1.03 × 10-4 s-1
(75.0 °C). g Other specific rate constants were 3.54 × 10-7 s-1 (50.0
°C), 6.07 × 10-6 s-1 (75.0 °C), and 7.18 × 10-5 s-1 (100.0 °C).
Solvolysis Ra tes. The rates of solvolysis at 25 °C of
12 2-aryl-2-chloro-4,4-dimethylpentanes (3) were deter-
mined in 90% acetone-10% water (v/v), the solvent that
had been used for determination of σ+ values on the basis
of solvolysis rates of 1;1 the specific rate constants are
shown in Table 2. The specific rate constants of 3g, 3i,
and 3l in various solvents (Table 3) were determined to
compare their Grunwald-Winstein type relations with
those of the corresponding substituted cumyl chlorides
1 that had been reported by Liu.2 The rates of 1l were
also measured in various solvents because of their
absence in previous studies (Table 3). To examine the
solvent effects on the F+ value, the rates of 1h , 1j, 3h ,
and 3j in TFE were determined and summarized in Table
4. The significance of these rate data is discussed in the
following sections.
h
i
Determined conductometrically. Other specific rate constants
were 2.27 × 10-6 s-1 (50.0 °C), 3.17 × 10-5 s-1 (75.0 °C), and 3.36
j
× 10-4 s-1 (100.0 °C). Other specific rate constants were 7.79 ×
10-6 s-1 (50.0 °C) and 1.09 × 10-4 s-1 (75.0 °C). ref 15. Other
k
l
specific rate constants were 0.0411 s-1 (-19.8 °C), 0.114 s-1 (-10.0
°C), and 0.191 s-1 (-5.0 °C). Other specific rate constants were
m
0.0178 s-1 (0.6 °C), 0.0412 s-1 (8.0 °C), 0.0506 s-1 (10.0 °C), and
n
0.0892 s-1 (15.0 °C). Other specific rate constants were 0.0251
s-1 (-0.4 °C), 0.0695 s-1 (9.1 °C), and 0.194 s-1 (20.0 °C). o Other
specific rate constants were 5.91 × 10-6 s-1 (50.0 °C) and 7.60 ×
10-5 s-1 (75.0 °C).
Ta ble 4. Sp ecific Ra te Con sta n ts (k, s-1) for th e
Solvolysis of 1h , 3h , 1j, a n d 3j in TF E a t 25 °Ca
substituent
Ha m m ett-Br ow n P lot for th e Ra tes in 90% Ac-
eton e. The 10 specific rate constants of solvolysis of 3
excluding 3k and 3l are plotted against Brown-Okamoto
σ+ constants in Figure 1. An excellent linear correlation
with F+ ) -4.51 (r > 0.999) was obtained for a σ+ range
from -0.46211 (p-cyclopropyl) to 0.7901 (p-NO2), suggest-
ing that the Brown-Okamoto σ+ values may be satis-
factorily used in rear-side shielded, open-chain systems
such as 3. In other words, there is no worry about an
undesirable perturbation of the σ+ values by solvent
nucleophilicity in the cumyl system, at least in the above
σ+ range.
The good Hammett-Brown relation in the solvolysis
of 3 indicates similarity in resonance demand in the
activated complexes for 1 and 3. This is experimentally
substantiated by p-CF3/m-CF3 rate ratios12 for 1 and 3
very close to each other, 0.37513a and 0.418, respectively,
in 90% acetone at 25 °C. Similarly, the respective p-CH3/
m-CH3 rate ratios are also close to each other at 13.013b
substrate
m-CF3 (h )
p-NO2 (j)
1
3
3.56 × 10-3 b
3.24 × 10-5 d
5.8 × 10-1 c
6.87 × 10-3 b
a
Determined in a single run conductometrically or titrimetri-
cally in the presence of 2 × 10-4 or 0.025 mol L-1 2,6-lutidine,
respectively. b Determined conductometrically. c Extrapolated from
conductometric specific rate constants 3.38 × 10-2 s-1 (-0.5 °C),
6.10 × 10-2 s-1 (4.5 °C), and 0.154 s-1 (12.5 °C). Determined
d
titrimetrically.
and 12.3. In contrast, the p-CF3/m-CF3 and p-CH3/m-CH3
rate ratios in a congested 2-aryl-2-chloro-3,3-dimethylbu-
tane system (4) are 0.862 and 8.44, respectively, in 80%
acetone at 45 °C,14 showing the diminished sensitivity
of the para substituents due to the decreased p-π overlap
in the twisted activated complex.
Previously, one of the present authors (K.T.) reported
that the σ+ value for the 3,5-bis(trifluoromethyl)phenyl
group was better described by 0.946 than by 2 × σ+
m-CF3
(1.040).15 However, the determination depended on the
(11) Hahn, R. C.; Corbin, T. F.; Shechter, H. J . Am. Chem. Soc. 1968,
90, 3403.
(12) Liu, K.-T. J . Chin. Chem. Soc. 1992, 39, 617.
(13) (a) Okamoto, Y.; Inukai, T.; Brown, H. C. J . Am. Chem. Soc.
1958, 80, 4969. (b) Brown, H. C.; Brady, J . D.; Grayson, M.; Bonner,
W. H. J . Am. Chem. Soc. 1957, 79, 1897.
(14) Fujio, M.; Nomura, H.; Nakata, K.; Saeki, Y.; Mishima, M.;
Kobayashi, S.; Matsushita, T.; Nishimoto, K.; Tsuno, Y. Tetrahedron
Lett. 1994, 35, 5005.
(15) Takeuchi, K.; Kurosaki, T.; Okamoto, K. Tetrahedron 1980, 36,
1557.

2378 J . Org. Chem., Vol. 64, No. 7, 1999
Takeuchi et al.
Ta ble 5. G+ Va lu es for th e Solvolysis of 1 a n d 3 in
Va r iou s Solven ts a t 25 °Ca ,b
F+
solventc
1
3
F+(3)/F+(1) ratio
100E
90E
80E
70E
-4.67d
-4.90
-4.95
-5.11
-4.54e
-5.46
-5.81
-5.91
-4.95
-5.25
-5.41
-5.53
-4.51
-5.62
-5.63
-5.63
1.06
1.07
1.09
1.08
0.99
1.03
0.97
0.95
90A
60T-40E
80T-20E
100T
a
The F+ values were obtained from m-Cl, p-CF3, and 3,5-(CF3)2
points except for the cases in 100E, 90A, and 100T. As the σ+ value
b
for 3,5-(CF3)2, +0.982 was used; see text. The correlation coef-
ficient was greater than 0.997. c For notations, see footnote b to
Table 3. Ref 17. e Ref 1.
d
structure, solvent, and magnitude of the F+ values. Very
recently, however, the Liu group has reported that the
F+ value for the solvolysis of substituted benzhydryl
chlorides changes from -4.10 to -4.26 in nucleophilic
solvents (EtOH, MeOH, their aqueous mixtures, and
aqueous acetone) to -4.93 (60T-40E) to -5.12 (80T-
20E) and that the nucleophilic solvent participation
F igu r e 1. The Hammett-Brown relationship for the solvoly-
sis of 2-aryl-2-chloro-4,4-dimethylpentanes (3) in 90% acetone
at 25 °C: F+ ) -4.51; r ) 0.999. The apparent σ+ values of
3,5-Cl2 and 3,5-(CF3)2 are evaluated to be 0.740 and 0.982,
respectively, by placing log k values for 3k (log k ) -5.507)
and 3l (log k ) -6.602) on the regression line.
decreases the sensitivity of solvolysis rates to σ+.19
A
similar trend has also been found in the solvolysis of
1-aryl-1-chloro-2,2-dimethylpropanes (1-aryl-1-tert-butyl-
methyl chlorides).20
In Table 5 are summarized the F+ values obtained for
the systems 1 and 3 in eight solvents. Most of the F+
values have been calculated from data derived from
compounds containing three substituents, m-Cl, p-CF3,
and 3,5-(CF3)2. The large negative F+ values in TFE,
-5.91 and -5.63 for the solvolysis of 1 and 3, respec-
tively, reflect the decreased nucleophilic (or Brønsted
base type) solvation by low nucleophilic solvent TFE. It
is noted that the ratio in F+ between 3 and 1 [F+(3)/F+-
(1)] is less than unity (0.95) in 100T, increases to 1.03 in
60T-40E, and becomes 1.07-1.09 in aqueous ethanol
solvents. The F+(3) less negative than F+(1) in TFE would
reflect an earlier transition state (TS) for 3 than for 1
because of a more congested destabilized ground state
for 3. The F+ for solvolysis of 1 less negative than that
for 3 in nucleophilic solvents would be ascribed to greater
nucleophilic (or Brønsted base type) solvation in the
activated complex for 1 that causes greater dispersion
of positive charge to solvent.
Although Tsuji et al. reported that the change of
solvents, such as aqueous acetone and aqueous ethanol,
does not cause very serious deviation in the linearity of
the Hammett-Brown plot,18 we wish to caution against
the use of the nitro substituent when TFE is used as
solvent. As shown in Figure 2, the p-NO2 data deviate
downward by approximately 0.5 log k unit in the Ham-
mett-Brown relation for both 1 and 3. Probably, highly
acidic TFE (pKa 12.3721) solvates the anionic oxygen of
the nitro group by hydrogen bonding, making it more
electron-withdrawing. Similar rationalizations for the
downward deviation of the solvolysis rates in TFE for the
Grunwald-Winstein relations of 2-oxo and 3-oxo bridge-
head compounds have been presented.22 By the same
estimated specific rate constant for the solvolysis of 3,5-
bis(trifluoromethyl)cumyl chloride in 90% acetone at 25
°C that was obtained by extrapolation from data at 75
and 100 °C. In the present work, the additivity of the
σ+
and σ+
constants was examined by placing
m-CF3
m-Cl
the log k values for 3k and 3l on the regression line in
Figure 1 and by reading the corresponding abscissa
values. In this way, the σ+ values for the 3,5-bis-
(trifluoromethyl) and 3,5-dichloro substituents were de-
termined to be 0.982 and 0.740, respectively, which are
smaller than 2 × σ+
(1.040) and 2 × σ+
(0.798)
m-CF3
m-Cl
by 0.058 σ+ unit in both cases. We recommend the use of
the newly determined σ+ values for the 3,5-(CF3)2 and
3,5-Cl2 substituents in 90% aqueous acetone, especially
when careful interpretation of rate data is required.
In this context, Creary reported a σ+ value of 0.701 for
the 3,5-dichloro substituents in EtOH.16 This value is also
smaller by 0.053 than 2 × σ+
(0.754) using Brown-
m-Cl
Okamoto’s σ+
in EtOH.17 Similarly, Tsuji and co-
m-Cl
workers estimate a σ+ value for the 3,5-dichloro substit-
uents in 80% aqueous acetone as 0.739,18 which is
practically identical to the present estimation of 0.740
in 90% aqueous acetone.
Solven t Effect on Rea ction Con sta n t G+. The effect
of solvent on the magnitude of F+ in the solvolysis of the
cumyl chloride system was studied by Brown in 1958 by
changing the solvent, e.g., MeOH (F+ ) -4.82), EtOH
(F+ ) -4.67), and i-PrOH (F+ ) -4.43), with 90% acetone
(F+ ) -4.54) being standard.17 Liu examined eight
solvents in the solvolysis of 2-aryl-2-chloroadamantanes
and obtained respective F+ values,2b but little attention
has been paid to the relation between the substrate
(16) Creary, X. J . Am. Chem. Soc. 1981, 103, 2463.
(17) Okamoto, Y.; Inukai, T.; Brown, H. C. J . Am. Chem. Soc. 1958,
(19) Liu, K.-T.; Lin, Y.-S.; Tsao, M.-L. J . Phys. Org. Chem. 1998,
11, 223.
80, 4972.
(20) Liu, K.-T.; Duann, Y.-F.; Yu, D.-G. J . Chin. Chem. Soc. 1998,
45, 153.
(18) Tsuji, Y.; Fujio, M.; Tsuno, Y. Bull. Chem. Soc. J pn. 1990, 63,
856.
(21) Ballinger, P.; Long, F. A. J . Am. Chem. Soc. 1959, 81, 1050.

Solvolysis of 2-Aryl-2-chloro-4,4-dimethylpentanes
J . Org. Chem., Vol. 64, No. 7, 1999 2379
F igu r e 2. The Hammett-Brown relationships for the sol-
volysis of some substituted cumyl chlorides (1) and 2-aryl-2-
chloro-4,4-dimethylpentanes (3) in TFE at 25 °C. The p-nitro
points are not included in the regression analysis: open circle
(1); solid circle (3).
F igu r e 3. A plot of log k values for 2-chloro-2-(p-trifluoro-
methyl)phenyl-4,4-dimethylpentane (3i) against those for
2-chloro-2-(p-trifluoromethyl)phenyladamantane (5) for sol-
volysis in various solvents at 25 °C. For the specific rate
constants for 3i and 5, see Table 3 and ref 2b, respectively.
token, the recent data reported by Liu and co-workers19
for the solvolysis of substituted benzhydryl chlorides
appears to show that p-nitrobenzhydryl chloride solvo-
lyzes approximately 2 times slower in 60T-40E than
expected from the rate in 80E.23
Com p a r ison of Ca tion Sta biliza tion by Solven t
betw een 1 a n d 3. Liu and co-workers concluded that
the solvolysis of cumyl chloride occurs with significant
nucleophilic intervention of solvent,2 whereas the Richard
group suggested that Lewis base type stabilization of
carbocations by nucleophilic solvation would be less
significant than the “solvation of Brønsted acids and
bases by formation of hydrogen bonds”.3 Both the Liu and
Richard groups showed that cationic solvation of the
cumyl system increases with increase in electron with-
drawing ability of substituent.
Recently, we suggested that the placement of one or
two neopentyl substituents in secondary and tertiary
alkyl substrates not only effectively shields the rear side
of the reaction center but also decreases the Brønsted
acid-base type solvation.5c To demonstrate that 3 is a
nearly limiting SN1 system, the values of log k for 3i were
plotted against those for 2-chloro-2-[(p-trifluoromethyl)-
phenyl]adamantane2b (5) that had been reported by Liu
(Figure 3). The good straight line shows that system 3
may be regarded as a limiting SN1, open-chain aralkyl
system.
the TFE (100T), 80T-20E, and 60T-40E points from the
aqueous ethanol and aqueous acetone line for the loga-
rithmic plot of the rate constants for the cumyl system
against those for 2-chloro-2-(m-chlorophenyl)adaman-
tane, i.e., YBnCl.2 The greater deviations were interpreted
as showing greater nucleophilic intervention by solvent.
It appeared to us, however, that it would be more
appropriate to compare the rates between the above two
systems 1 and 3 having the same substituent. In the
present work, we compared the rates of the cumyl
system2f 1 with those of 3 for same substituents, m-Cl,
p-CF3, and 3,5-(CF3)2. Figure 4 shows the plots of
log(k/k0) between the two systems for these substituents,
where k0 is the specific rate constant for solvolysis in 80%
ethanol. Apparently, the downward deviation of less
nucleophilic solvents containing TFE becomes larger in
the order m-Cl, p-CF3, and 3,5-(CF3)2. Liu and co-workers
showed that replacement of one methyl group of 1e with
an isopropyl group essentially eliminates the nucleophilic
solvent intervention, but it revives for the m-chloro
derivative.24 The present result is consistent with their
conclusion that the nucleophilic intervention by solvent
in the cumyl and related systems increases as the
carbocation becomes more electron-demanding.2 Analyses
of the rate data in Table 3 for the solvolysis of 3g, 3i, 3l,
and 1l and reported data2e,f for 1g and 1i by the extended
Nucleophilic intervention by solvent was evaluated by
the Liu group by comparing the downward deviations of
(22) (a) Yoshida, M.; Takeuchi, K. J . Org. Chem. 1993, 58, 2566. (b)
Takeuchi, K.; Yoshida, M.; Kurihara, Y.; Kinoshita, T.; Kitagawa, T.
J . Org. Chem. 1994, 59, 6459. (c) Takeuchi, K.; Ohga, Y.; Yoshida, M.;
Ikai, K.; Shibata, T.; Kato, M.; Tsugeno, A. J . Org. Chem. 1997, 62,
5696.
(23) This conclusion was drawn on the basis of the downward
deviation of the p-NO2 data point in a plot of log k in 60T-40E vs log
k in 80E for the solvolysis of p-CH3, (H), m-Cl, m-CF3, and p-NO2
benzhydryl chlorides by using the reported data;19 see Figure S1 in
Supporting Information.
Grunwald-Winstein relationship, log(k/k0) ) mYBnCl
+
lNOTs, show that 3 compounds are less nucleophilically
assisted than 1 compounds by 0.2-0.4 l unit.25 However,
present data do not permit evaluation of the relative
(24) Liu, K.-T.; Chen, P.-S. J . Chem. Res., Synop. 1994, 310.

2380 J . Org. Chem., Vol. 64, No. 7, 1999
Takeuchi et al.
(1.30 g, 53 mmol) in diethyl ether: bp 58-59 °C at 0.18 mmHg.
3i-OH (3.98 g) was treated with dry HCl in pentane at -78
°C for 30 min, the reaction mixture was dried over CaCl2, and
the solvent was evaporated to give a yellow oil that was found
to contain 3i in 97% and 2-[(p-trifluoromethyl)phenyl]-4,4-
dimethyl-1-pentene in 3%. The mixture was used for rate and
product studies.
2-Ch lor o-2-(m -ch lor op h en yl)-4,4-d im eth ylp en ta n e (3g)
by Hyd r och lor in a tion of Olefin ; Typ ica l P r oced u r e. The
corresponding alcohol, 2-(m-chlorophenyl)-4,4-dimethylpen-
tanol (3g-OH), was prepared in 79% yield by treating 4,4-
dimethyl-2-pentanone (1.93 g, 13.4 mmol) with m-chlorophe-
nylmagnesium bromide prepared from m-bromochlorobenzene
(3.57 g, 18.6 mmol) and Mg turnings (0.58 g, 24 mmol) in
diethyl ether: bp 74-76 °C at 0.23 mmHg. 3g-OH (3.17 g)
was mixed with KHSO4 (3.26 g), and the mixture was heated
in a distillation flask to give essentially pure 2-(m-chlorophe-
nyl)-4,4-dimethyl-1-pentene (2.86 g) as a colorless liquid in 83%
yield: bp 68-69 °C at 0.28 mmHg. The olefin was treated with
dry HCl in CH2Cl2 at 0 °C to give 3g as a pale yellow oil. A
13C NMR measurement showed the purity of 3g to be 84%,
the rest being the starting olefin.
Ra te Stu d ies. The rates of solvolysis were determined
titrimetrically or conductometrically following previously re-
ported procedures.27 The chlorides obtained by hydrochlori-
nation of the corresponding tertiary alcohols or olefins were
used without further purification because of the instability of
the chlorides. The first-order plots were satisfactorily linear
until 80-90% conversion.
P r od u ct Stu d ies for th e Solvolysis of 3i. A solution of
3i (279 mg, 10.0 mmol) in 90% (v/v) aqueous acetone (50.0 mL)
containing 0.025 mol L-1 of 2,6-lutidine was kept at 25.0 °C
in a constant-temperature bath for 168 h (10 half-lives). Most
of the acetone was rotary evaporated, and the residue was
extracted with diethyl ether. The organic layer was washed
with 5% aqueous HCl and 10% aqueous Na2CO3 and dried
(MgSO4). After evaporation of the solvent, the residue was
F igu r e 4. Plots of log(k/k0) values for 1g, 1i, and 1l versus
those for 3g, 3i, and 3l, respectively, for solvolysis in various
solvents at 25 °C. For the specific rate constants, see Table 3
for 3g, 3i, 3l, and 1l and ref 2f for 1g and 1i: solid circle (1g
vs 3g); solid square (1i vs 3i); solid triangle (1l vs 3l).
importance of the Brønsted base type solvation to the
methyl hydrogen atoms of the cumyl cation and the Lewis
base type solvent intervention toward the cationic carbon.
1
analyzed by H NMR (270 MHz, CDCl3) to show the presence
of 2-[(p-trifluoromethyl)phenyl]-4,4-dimethyl-2-pentanol (3i-
OH) (29%) and 2-[(p-trifluoromethyl)phenyl]-4,4-dimethyl-1-
pentene (66%). The weak absorptions at δ 2.16 suggested that
(Z)- and (E)-2-[(p-trifluoromethyl)phenyl]-4,4-dimethyl-2-pen-
tenes had been formed in 5%.
Exp er im en ta l Section
Boiling points are uncorrected. 2,6-Lutidine was distilled
over CaH2. Other commercially available reagents were re-
agent grade quality and were used as received. Solvolysis
solvents were prepared as previously described.27 2-Aryl-2-
chloropropanes, 1h ,13a 1j,28 and 1l,15 were prepared following
the reported procedures. The 13C NMR and boiling point data
for 2-aryl-4,4-dimethyl-2-pentanols (3-OH) are summarized in
Supporting Information. Preparation of 2-chloro-2-(p-nitro-
phenyl)-4,4-dimethylpentane (3j) and 13C NMR data for 2-aryl-
2-chloro-4,4-dimethylpentanes (3) are also given in Supporting
Information.
2-Ch lor o-2-[(p -t r iflu or om et h yl)p h en yl]-4,4-d im et h yl-
p en ta n e (3i) by Hyd r och lor in a tion of Alcoh ol (3i-OH);
Typ ica l P r oced u r e. The alcohol, 2-[(p-trifluoromethyl)phe-
nyl]-4,4-dimethylpentanol (3i-OH), was prepared in 63% yield
by treating 4,4-dimethyl-2-pentanone (4.97 g, 43.4 mmol) with
p-(trifluoromethyl)phenylmagnesium bromide prepared from
p-bromobenzotrifluoride (11.4 g, 50.7 mmol) and Mg turnings
Com p u ta tion a l Stu d ies. Ab initio molecular orbital cal-
culations were carried out with the Gaussian 94 program on
a
CRAY T94/4128 computer. The geometries were fully
optimized at the RHF/6-31G* level for all the molecules
treated.
Ack n ow led gm en t. The authors are grateful to
Professor Kwang-Ting Liu for valuable suggestions.
Computation time was provided by the Supercomputer
Laboratory, Institute for Chemical Research, Kyoto
University. This work was supported in part by a Grant-
in-Aid for Scientific Research (10440188) from the
Ministry of Education, Science, Sports and Culture,
J apan, which is gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: 13C NMR and boil-
ing point data for 2-aryl-4,4-dimethyl-2-pentanols (3-OH),
synthetic procedures for 2-chloro-2-(p-nitrophenyl)-4,4-dim-
ethylpentane (3j), 13C NMR data for some 2-aryl-2-chloro-4,4-
dimethylpentanes (3e-l), a plot of log k in 60T-40E vs log k
in 80E for the solvolysis of various benzhydryl chlorides
(Figure S1), and some analysis results for the extended
Grunwald-Winstein relation. This material is available free
of charge via the Internet at http://pubs.acs.org.
(25) For detailed analysis results, see Supporting Information. The
use of NT in place of NOTs led to a similar conclusion. For YBnCl, NOTs
and NT values, see refs 2b, 26, and 4a, respectively.
,
(26) Bentley, T. W.; Llewellyn, G. Prog. Phys. Org. Chem. 1990, 17,
121.
(27) Takeuchi, K.; Ikai, K.; Shibata, T.; Tsugeno, A. J . Org. Chem.
1988, 53, 2852.
(28) Okamoto, Y.; Brown, H. C. J . Am. Chem. Soc. 1957, 79, 1909.
J O9821067